piston stop tool - adjustment?

[elorenzof]

Why does the piston stop tool have an adjustment? How do you know how far to adjust it in to correctly find top dead center on cyl # 1? It seems to me if i adjust it in and the piston touches it, then you have tdc but if you screwed it out then the piston would raise more and touch it again, then you have tdc. am i complicating things? Basically i know you screw it into the cyl # 1 but how do i adjust the inner portion (how far) to find tdc